Image sizes

Check that uploaded images can use all built-in and custom image sizes (e.g. sizes added with `add_image_size`) in the theme. There is a suspicion that under some circumstances the ‘physical’ image files are created, but the metadata pointing to them is lost and so the files linked on the page in the image `src` attribute is the full size image.

Reproduce the issue with uploaded images using built-in sizes and a custom size added with add_image_size. Trace whether the physical files are created and whether their metadata is retained, then verify that the page image src uses the requested size rather than the full-size image.

Written by the indexing model from the issue text.